1. Field of the Invention
The present invention relates generally to methods for estimating and measuring longitudinal dispersion in optical waveguide fibers.
2. Technical Background
Optical fiber has become a favorite medium for telecommunications due to its high capacity and immunity to electrical noise. However, chromatic dispersion can limit the bandwidth and the reach of optical fiber by producing pulse spreading due to various wavelengths of light traveling through the optical fiber at different speeds. Thus, different signal wavelengths arrive at a destination at different times, therefore causing the transmitted pulse to spread or “disperse”, as it travels through the optical fiber.
Chromatic dispersion includes two components—one due to the core material's property and one due to waveguide dispersion of the fiber (defined by fiber's refractive index profile). One characteristic of dispersion is dispersion slope. Dispersion slope is a ratio of dispersion change with a change in wavelength. Dispersion slope can be either positive, or negative. A typical transmission fiber has a positive dispersion slope. A typical dispersion compensating fiber has a negative dispersion slope at the transmission wavelength. Dispersion value of a fiber, at a specific wavelength, can be either positive or negative. It is zero at the point where the dispersion curve crosses the horizontal (wavelength) axis. The wavelength at which the chromatic dispersion is zero is known as zero dispersion wavelength. A typical transmission fiber, such as SMF-28® has a zero dispersion wavelength of about 1310 nm. A typical dispersion shifted fiber has a zero dispersion wavelength of about 1550 nm. Other optical fibers have different zero dispersion wavelengths.
The need to satisfy demand for transmission capacity has led to dispersion management, which includes the use of dispersion compensating or dispersion managed fiber segments to compensate for dispersion introduced by the typical transmission fiber, for example SMF-28®. More specifically, the dispersion compensating fiber has dispersion and dispersion slope that are of opposite sign than that of the transmission fiber, which results in zero or nearly zero dispersion in the operating wavelength band.
To achieve this goal, the segment of dispersion compensating fiber (or dispersion shifted fiber) that is being coupled to transmission fiber must have a predetermined dispersion slope, and dispersion sufficient to compensate for the specific length of the transmission fiber. However, dispersion parameters can vary along the fiber length and from fiber to fiber, even among fibers of the same type. Thus, it is difficult to know before hand the precise length of the dispersion compensating fiber (or the length of the dispersion managed fiber segment) that is needed to compensate for dispersion introduced by the transmission fiber.
It is well known that the physical properties of the optical fibers can vary as the fiber is being drawn. This influences optical properties, especially dispersion. For any particular length of optical fiber, end-to end dispersion value can be easily measured, and the average dispersion (ps/nm/km) can be easily determined from this measurement. However, because dispersion parameters can vary as a function of fiber segment length, cutting the dispersion compensating fiber segment in half, for example, will typically provide either more or less than half of the total measured dispersion value. Thus, variability of dispersion parameters within the length of the same fiber makes it difficult to predict the precise length of the fiber segment that needs to be cut off from the long length of fiber to effectively compensate for chromatic dispersion introduced by transmission fiber.
One way to address this problem is to have a dispersion “map” that provides chromatic dispersion data along the length of the fiber or along the optical fiber span (which may include several spliced optical fibers). Several techniques have been developed to measure chromatic dispersion and zero dispersion wavelength variations along the lengths of the fiber. One technique based on the use of Optical Time Domain Reflectometry (OTDR) has been proposed for step-index fibers as described in an article entitled “Novel Technique for Measuring the Distributed Zero-Dispersion Wavelength of Optical Fibers”, Electronics Letters 29, 426 (1993) by M. Ohashi and M. Tateda, incorporated by reference herein. This article teaches that if the doping of the fiber preform does not change over its length (no change in core composition), then the changes in the zero dispersion wavelength (λ0) of the resulting fiber are due to the changes in the refractive index profile (i.e., due to changes in core size). This article discloses how to estimate dispersion as a function of longitudinal position, by knowing the fiber core's refractive index and by measuring the optical fiber's longitudinal mode field diameter (MFD) as a function of wavelength. The disclosed technique utilizes bidirectional OTDR data at multiple wavelengths to exactly determine the dependence of MFD upon wavelength. Thus, this approach requires extensive, time consuming data gathering. In addition, the disclosed technique requires the use of two or more OTDR laser sources. In addition, these methods require the use of pigtail (i.e., reference) fibers with characteristics that are very similar to that of the measured fiber.
Other reported techniques rely on the use of four-wave mixing (FWM) as a probe for the chromatic dispersion (D) or zero-dispersion wavelength (λ0) fluctuation (see, for example, Y. Suetsugu, T. Kato, T. Okuno, and M. Nishimura, IEEE Phot. Lett. 7, 1459 (1995); S. Nishi and M. Saruwatari, Electron. Lett. 32,579 (1996); and M. Eiselt, R. M. Jopson, and R. H. Stolen, J. Lightwave Technol. 15, 135 (1997), all of which are incorporated by reference herein). The four wave mixing techniques to measure dispersion characteristics as a function of fiber lengths are described U.S. Pat. Nos. 5,956,131; 6,067,149; 5,619,320; and 6,118,523. However, these approaches also require extensive and time consuming data gathering with specialized equipment that operates at high power levels (so that the fiber is operating in a non-linear regime).
Therefore, there is a need to obtain a versatile method for measuring the change in dispersion as a function of longitudinal position in an optical fiber, without the need for multiple wavelengths measurements and without the need for additional equipment or equipment modification.